1. Field of the Invention
The present invention relates to a semiconductor memory employing electrically rewritable memory cells. More specifically, the present invention relates to a semiconductor memory which constitutes memory cell units by connecting memory cells in series.
2. Related Background Art
Normally, an EEPROM memory cell has a MISFET structure in which a charge accumulation layer and a control gate are layered. This memory cell stores data in a nonvolatile manner based on the difference between a threshold voltage at the state of injecting electric charge into the charge accumulation layer and a threshold voltage at the state of discharging the electric charge. The injection and emission of charges are carried out by a tunnel current through a tunnel insulating film provided between the charge accumulation layer and a substrate channel.
Among EEPROM's, a so-called NAND type EEPROM which constitutes NAND cell units by connecting a plurality of memory cells in series requires fewer selected transistors than an NOR type EEPROM, thereby realizing higher integration.
In order to read data from the NAND type EEPROM, a read voltage for threshold voltage determination is applied to the control gate of a selected memory cell in the NAND cells, a pass voltage higher than the read voltage for turning on the memory cell irrespective of data is applied to the control gates of the remaining unselected memory cells and a current penetrating the NAND cell string is detected. Therefore, even if the same data is written, the read current differs according to the state of data of the unselected memory cells and the position of the selected memory cell in the NAND cells. Further, since data is read according to the quantity of charges which pass the current terminals of the memory cells, the apparent threshold voltage of each memory cell disadvantageously changes.
The generation of the difference in read current according to the data states of the unselected memory cell and the position of the selected memory cell will be specifically described with reference to FIGS. 41 to 43. FIGS. 41 and 42 show different read conditions for a NAND cell unit constituted by connecting 16 memory cells M0 to M15 in series, respectively. One end of a NAND cell is connected to a data transfer line (bit line) BL through a select transistor S1 and the other end thereof is connected to a common source line SL having a reference potential through a select transistor S2. The control gate of the respective memory cells M0 to M15 are connected to different data control lines (word lines) WL0 to WL15, respectively and the gates of the select transistors S1 and S2 are connected to select gate lines SSL and GSL for block selection, respectively.
While each of FIGS. 41 and 42 shows only one NAND cell unit, a plurality of NAND cell units of this type are arranged in a bit line direction and a word line direction and a memory cell array is thereby formed. In addition, a sense amplifier/data register is connected to the bit line BL. In a flash memory, the range of a plurality of NAND cell units aligned in the word line direction serves as a block which is a unit of batch erasure of data. Description will be given hereinafter while assuming that a state, in which electrons are discharged from the charge accumulation layer and a threshold voltage is low, is a “1” data state (erasure state) and a state, in which electrons are injected into the charge accumulation layer and a threshold voltage is high, is a “0” data state.
FIGS. 41A and 41B show the read voltages when the memory cell M0 closest to the bit line BL among the memory cells M0 to M15 is selected. In this case, the common source line SL has a ground potential GND, the bit line BL is applied with, for example, a positive voltage VBL of about 1V, the selected word line WL0 is applied with a read voltage Vr for threshold voltage determination and the remaining unselected word lines WL1 to WL16 are each applied with a pass voltage Vread necessary to turn on a cell irrespective of the data. In addition, each of the select gate lines SSL and GSL is applied with the pass voltage Vread, as well.
FIG. 43 shows an example of the threshold voltage distribution of a memory cell which stores binary data. The upper limit Vthw of the threshold voltage of the “0” data is set at 2V, the upper limit Vthe of the threshold voltage of the “1” data (erasure state) is set at −1V and the pass voltage Vread is set at a voltage between 4 to 5V, for example. In addition, the read voltage Vr is set at, for example, 0V. While FIG. 43 shows the threshold voltages of the select transistors S1 and S2, they are lower than the upper limit Vthw of the write threshold voltage of a memory cell. By applying the pass voltage Vread to the select transistors S1 and S2 have, therefore, the select transistors S1 and S2 have higher conductance than that of each memory cell or are sufficiently kept conductive.
FIG. 41A shows that the selected memory cell M0 has “1” data and each of the remaining unselected memory cells M1 to M15 has “1” data, as well. FIG. 41B shows that the selected memory cell M0 has “1” data and each of the remaining unselected memory cells M1 to M15 has “0” data. In these two cases, the relationship between read currents ID1 and ID2 which are carried to the NAND cell unit satisfies ID1>ID2. This is because the case shown in FIG. 41B is higher than that shown in FIG. 41A in the resistance between the source and the drain of each of the unselected memory cells M1 to M15.
FIGS. 42A and 42B show the relationship among read voltages in the case where the memory cell M15 closest to the common source line SL of the NAND cells is selected. FIG. 42A shows that each of the memory cells M0 to M15 has “1” data and FIG. 42B shows that the selected memory cell M15 has “1” data and each of the remaining unselected memory cells M0 to M14 has “0” data. In these cases, if the voltage VBL is lower than (Vread−Vthw), each of the memory cells M0 to M14 operates in a linear region. The case shown in FIG. 42B is higher in series resistance than the case shown in FIG. 42A. In addition, the memory cell M15 also operates in the linear region and the voltage between the drain and the source of the memory cell M15 is low. Further, the relationship between read currents ID3 and ID4 shown in FIGS. 42A and 42B, respectively, satisfies ID3>ID4.
If the substrate bias effect of each memory cell is taken into account, the memory cell M0 closer to the data transfer line BL is applied with a higher substrate bias than that of the memory cell M15 closer to the common source line SL. As a result, ID2 becomes lower than ID4 and ID1 becomes lower than ID3.
A problem in which the threshold voltage at the erasure state rises occurs by carrying out the erasure, write and read sequences, as shown in FIGS. 44A and 44B, even if the same data is written. Hereinafter, such a problem will be described.
In FIG. 44A, the memory cells M0 to M15 in the NAND cell unit are batch-erased and each is set in a “1” data state (in a step SE1). In a step SE2, the data of the memory cell M0 is read in the voltage relationship shown in FIG. 41A and it is determined whether the data is “0” or “1” at a constant current level Ith. Alternatively, the data may be determined not by applying the constant current Ith but by, for example, precharging the data transfer line with VBL to turn the data transfer line into a floating state, reading the data and detecting the potential change of the data transfer line using a sense amplifier. Further, “0” data is written to each of the memory cells M1 to M15 and the threshold voltage thereof is raised (in a step SE3). Next, in a step SE4, the data of the memory cell M0 is read in the voltage relationship shown in FIG. 41B and it is determined whether the data thus read is “0” or “1” at the constant current level Ith.
In this sequence, even in the memory cell M0 in the same erasure state, the read currents ID1 and ID2 described with reference to FIGS. 41A and 41B differ from each other. Due to this, it is determined in the step SE4 the read current ID2 is not higher than the determination current Ith. Besides, in the step SE2, the read current ID1 becomes higher than the determination current Ith. In other words, the threshold voltage distribution in the step SE4 is raised toward a positive threshold voltage compared with that in the step SE2 at the same current level and statuses indicated by a dashed line and a solid line occur as shown in FIG. 43.
In FIG. 44B, the memory cells M0 to M15 of the NAND cell unit are batch-erased to set to the “1” data state (in a step SE1′). In a step SE2′, the data of the memory cell M15 is read in the relationship shown in FIG. 42A and it is determined whether the data is “0” or “1” at the constant current level Ith. Further, “0” data is written to each of the memory cells M0 to M14 to raise the threshold voltage thereof (in a step SE3′). In a step SE4′, the data of the memory cell M15 is read in the voltage relationship shown in FIG. 42B and it is determined whether the data is “0” or “1” at the constant current level Ith.
In this sequence, even in the memory cell M15 in the same erasure state, the read currents ID3 and ID4 described with reference to FIGS. 42A and 42B differ from each other. Due to this, the read current ID4 becomes not higher than the determination current Ith in the step SE4′ and the read current ID3 becomes higher than the determination current Ith in the step SE2′. In this case, the threshold voltage distribution in the step SE4′ is raised toward a positive threshold voltage compared with that in the step SE2′ at the same current level and statuses indicated by a dot line and a solid line occur as shown in FIG. 43.
On the other hand, when the same logical value is read, if the read current of the memory cell greatly changes according to the position of the memory cell and the data of the unselected memory cells, it is difficult to shorten maximum read time and to reduce the maximum electromagnetic noise generated by the cell current. This is because the maximum read time is determined by the condition that the read current of a selected cell becomes the lowest and the maximum electromagnetic noise is determined by the condition that the read current of the selected memory cell becomes the highest.
If the memory cell read current is higher further, the floating of the potential of the common source line increases. If write and verification read operations are repeatedly carried out, defect in which “0” data cannot be sufficiently written occurs (see, for example. Japanese Patent Application Laid-open Publication No. 11-260076). In addition, since the maximum current flowing through the data transfer line increases, the rise of wiring resistance and the deterioration of reliability due to electro-migration caused by current stress, the change of the threshold voltage of the transistor and the increase of leakage current following the increase of heat emission disadvantageously occur.
Furthermore, if the threshold voltage of “1” data becomes higher, the difference between the lower limit of the threshold voltage distribution of the “0” data and the upper limit of the threshold voltage distribution of the “1” data decreases. As a result, the probability of, for example, erroneously reading “1” data as “0” data increases. To eliminate the reading error, it is necessary to widen the threshold voltage distribution of, for example, the “0” data toward a higher region. This in turn brings about another disadvantage. That is, the holding characteristic of holding data having a high threshold voltage is inferior to that of data having a low threshold voltage because of the self electric field of accumulated charges. Due to this, if the threshold voltage distribution of the “0” data ranges toward the too high region, it is difficult to obtain sufficient data holding characteristic. In addition, it is necessary to apply a higher pass voltage than the maximum voltage of the threshold voltage distribution to unselected memory cells in the NAND cell unit during reading. Therefore, if read operation is repeated, negative charges are accumulated in the charge accumulation layer to raise the threshold voltage and further increase the upper limit of the threshold voltage in the erasure state. This disadvantageously causes data destruction and reading error.
As described above, according to the conventional NAND type EEPROM, read current differs depending on the data states of the unselected memory cells and the position of the selected memory cell in the NAND cell unit, whereby various disadvantages to the improvement of the EEPROM, such as reading errors and data destruction, occur.